1. Field of Invention
The present invention relates generally to injection molding systems. More particularly, the invention relates to a hot sprue bushing and to hot runner injection molding apparatus providing improved thermal gate control.
2. Summary of the Prior Art
Hot runner, or so-called "runnerless" injection molding apparatus are well known in the art. Basically, these apparatus may be classified in three distinct categories, namely, (1) insulated, (2) internally heated (sometimes referred to as "modified insulated"), and (3) "true" hot runner (sometimes referred to as "open bore hot manifold").
The insulated runner apparatus was the first apparatus which allowed a molder to produce parts without sprues or runners attached thereto. In basic design, this type of apparatus includes a channel milled between two plates located in stacked relation to one another adjacent to one side of the mold. This channel is continued to locations abutting the mold. At these locations, small gates (holes) are provided between the channel ends and the respective article formation cavities of the mold.
In operation, the channel is filled with plastic material in molten form. The material adjacent to the outer portions of the channel solidifies so as to insulate the material in the center of the channel from the comparatively cold metal forming the channel walls. Hence, so long as a flow of hot material is maintained through the central portion of the channel, sufficient heat is continuously introduced into the system to keep the runner flowing.
This apparatus is very sensitive to cycle interruptions and/or overly long cycle times. This is because the runner will solidify in the absence of a substantially continuous input of heat supplied by a flow of molten material therethrough. More particularly, the cycle of an injection molding machine includes (a) a high melt pressure injection portion; and (b) a subsequent portion during which the applied melt pressure is reduced. During the latter portion of the cycle, the molded article is solidified, the mold opened, the article ejected and the mold reclosed. It, therefore, will be understood that the insulated apparatus is limited to the manufacture of thin walled articles (i.e., articles having a small, comparatively fast solidifying volume) in a fast cycling mold. In addition, the length of the various runner channels must be comparatively short in order to avoid "freeze off" during the low melt pressure interval. Accordingly, such apparatus are limited to use with molds containing a limited number of cavities. In view of these drawbacks, the insulated apparatus is not currently in common use by production molders.
The internally heated apparatus adds heaters (commonly of the electrical resistance type) within the runner channels and also substantially adjacent to the gate area of an insulated injection molding apparatus. The addition of these heaters alleviates the problems of runner and/or gate freeze-off during cycle interruptions in insulated apparatus. Several other problems are created, however.
Specifically, since electrical resistance heaters inherently display a temperature gradient along their lengths, their use creates regions which are hotter and regions which are colder than the desired operating temperature of the apparatus along the runner path. Accordingly, it is necessary to adjust the temperature output from the colder regions so as to assure that melt freeze off will not occur in those regions. As a result, however, the temperature of the hotter regions is also increased. This may cause material degradation or burning in the hotter regions. In addition, the apparatus is inefficient in terms of energy input.
Further, material flow through such internally heated apparatus has been found to be non-uniform. In particular, the need for the melt to flow in annular channels surrounding the heater elements creates undesirable pressure losses along the length of the runner channels.
In the "true" hot runner apparatus, the material in the runner channels is substantially continuously heated from outside the melt flow. This is accomplished by heating a manifold containing the runner channel(s) under the control of thermocouples. The temperature of the metallic manifold, therefore, may be maintained at the desired melt temperature.
Such "true" hot runner apparatus allow flexibility in runner channel sizing, high material flow to surface contact ratios and the control of melt flow balance. Specifically, melt flow balance to numerous article formation cavities in the mold may be achieved (i) by the design of even numbers of branching runner channels in the heated manifold, and/or (ii) by the control of gate orifice cross-sections so as to balance the pressure utilized to deliver the melt to each article formation cavity of the mold. Hence, such "true" hot runner apparatus (1) may process substantially all thermoplastic materials, (2) may supply melt material to several article formation cavities simultaneously, and (3) may be made to mold diverse part volumes--all without adverse consequences to article quality or apparatus operation.
"True" hot runner apparatus also generally include elongate drops or bushings. These elements define central runner channels which communicate with the manifold runner channels. More particularly, the bushings are sealingly connected to the hot manifold and extend therefrom to locations adjacent to the critical gate areas of the mold. At these gate areas, the melt material flowing from the manifold runner channels through the bushings is ejected from the bushings, and injected into the article formation cavities of the mold through the gate.
Typically, the connection of the bushing to the manifold is formed by a threaded, surface compression fit or other rigid engagement of the upper end of the bushing with the lower portion of the manifold. This engagement of the bushing with the manifold also generally includes a seal to prevent flashing of the melt material between the adjoining hot surfaces of the manifold and the bushing. This seal commonly consists of a hollow, stainless steel, gasket-like O-ring. This O-ring resides in an annular space defined by a groove adjacent the interface of the lower surface of the manifold and an upwardly facing surface of the bushing.
The bushings are maintained at the melt temperature by appropriate heating means such as heater bands, coil heaters or heat pipes. In at least the latter of these alternatives, the bushing heat may be derived directly from the manifold so as to remain in balance therewith. Further, since both the manifold and the bushing are maintained at the melt temperature, means must be provided to substantially isolate those elements from the cold mold. Without such isolation means, the contact of the cold mold with the hot manifold would result in undesirable heat transfer to the mold. This, in turn, could detrimentally impact upon the article formation characteristics of the mold by slowing article solidification times. It would also result in the need for the provision of additional heat input to the metal surrounding the runner channel in order to avoid undesirable solidification of the melt material flowing therethrough.
Such insulation is generally accomplished by the provision of support pads located between the lower manifold surface and the upper mold surface. These support pads provide spaced structural contact between the hot manifold and the cold mold. They are also made of materials having a low thermal conductivity. Accordingly, an air gap is created between the lower surface of the manifold and the upper surface of the mold. That air gap and those support pads act to substantially thermally isolate the manifold from the mold.
In addition, air gaps and/or other insulating means are typically provided between the bushings and the mold. More particularly, the bushings commonly extend downwardly from the lower surface of the manifold, through the air gap created by the support pads, and thence into a bore extending into the upper surface of the mold. A small gate (opening) extends from the base of the bore to the article formation cavity of the mold. Further, the bushing is so sized and mounted that it is spaced from the side wall and base of the bore.
In this connection, it is well known in the art that the relative geometrical configurations of, and composition of, the bushing tip and the base of the bore in the mold are to be carefully controlled. These features allow the mold to pull heat from the gate area during reduced melt pressure portions of the injection molding cycle in a manner which produces an approximation of the desired, axially extending thermal gradient (profile) in the gate area. As will be described in greater detail below, it is this thermal profile in the gate area which allows the "vestige" to break without stringing or drooling.
In addition, features, such as (i) mounting slots for the proximal ends of the bushings in association with the manifold, or (ii) centering protrusions associated with the bore in the mold adapted to engage the bushing proximally of the gate area, are often incorporated into the overall apparatus design. The mounting slots allow the bushing to remain centered relative to the bore despite thermal expansion and/or contraction of the various elements of the apparatus. The centering protrusions, on the other hand, assure the alignment of the runner channel with the gate. The contact area between the protrusions and the bushing, however, is kept small. Further, that contact area is spaced from the gate area. These features minimize the detrimental effects of direct contact between the bushing and the mold.
In some cases, bushings have been developed which include a bore defining portion having a small gate at the bottom thereof. In such cases, the mold maker is relieved of the responsibility of creating components which must be machined to very close tolerances for receipt of the bushings. Instead, the mold is simply provided with roughly sized bores defining small gates to the article formation cavities at the bottom thereof. The bore defining portions of the bushings are then press-fit into, or otherwise secured within, the rough bores so as to effectively become part of the mold.
Thermal control in the gap between the output of the bushing runner channel and the gate input (the so-called "gate area") in "true" runnerless injection molding apparatus during the low melt pressure portion of the injection molding cycle is critical to the operation of the apparatus. Without such control, undesirable stringing of the plastic material at the break point between the molten material and the so-called "vestige" of the formed article, drooling of melt material from the gate output subsequent to formed article ejection from the mold and/or freeze-off of the gate area may take place. These events may result in ruined parts, material waste, the need for unnecessary trimming operations and/or undesirable machine down time to free the runner channel of frozen (i.e., solidified) melt material. Further, the size of the gate and melt injection pressure must be so selected that the article formation cavity of the mold may be completely filled without defects during an appropriately selected cycle time period. In addition, changes in the color of the melt should be possible without disassembly of the apparatus to remove material of the color previously injection molded thereby.
Several alternatives have been utilized heretofore in the art in an attempt to provide the required thermal gate area control in "true" runnerless injection molding apparatus. In each of these alternatives, the primary goal has been to control the axial and radial temperature gradient (profile) in the gate area during the low melt pressure portion of the injection molding cycle. Specifically, in the ideal case, the axial temperature gradient (profile) between the output of the bushing runner and the input of the gate subsequent to each injection "shot" of melt material should be such as to allow gradual cooling for a short distance from the runner output under the influence of the heated bushing. Thereafter, a sharp temperature drop followed by further gradual cooling under the influence of the cold mold should be provided. Such a temperature gradient (profile) allows the so-called "vestige" of melt material extending outwardly from the article formed by the mold to break cleanly in the gate area.
More particularly, the article should not draw a "string" of melt material therewith upon its removal from the mold. In addition, melt material should not "drool" from the gate after the removal of the article from the mold. At the same time, however, the gate orifice diameter must meet the article formation cavity fill requirements of the mold. Further, in order to avoid material clogging, the gate orifice must be sized so as to permit the passage of melt material therethrough without exceeding the allowable shear rate of the material being processed.
Since the resins which are typically injection molded fall into two general categories--Amorphous and Crystalline--it will be understood that no single gate design will effectively handle every material. Amorphous materials have no specific melting point, but become easier flowing with increases in temperature. They have a wide processing window and require a dramatic drop in temperature at the critical vestige break point region of the gate area. If this region is too hot, stringing and/or drooling will occur. If it is too cold, gate freeze-off will occur. Crystalline materials, on the other hand, have a sharply defined melting point. Accordingly, higher temperatures in the gate area must be maintained in order to avoid freeze-off. At the same time, however, too high temperatures must be avoided in order to prevent stinging and/or drooling.
In one of the above-mentioned alternatives to the solution of these problems, the melt material exiting the bushing under mold filling pressure is allowed to flow both (i) into the article forming cavity of the mold through the gate and (ii) into the gap between the bushing and the bore of the mold. In this case, the melt material injected into the space between the bushing and at least the lower portion of the bore acts as an insulator. Further, it has been found that by appropriately selecting the materials and configuration of the distal end of the bushing and the gate, the desired temperature gradient in the gate area may be approximated. In some cases, it has been found that computer modeling by a process commonly known as finite element analysis may be used to advantage in making such selections.
Melt material in the gap between the bushing and the bore, however, tends to degrade over time. Further, the melt material adjacent to the hot outer surface of the bushing remains in a substantially molten condition while that adjacent to the cold mold tends to solidify. Accordingly, molten, degraded material may migrate into the main melt stream flowing into the article formation cavity of the mold. This may ruin one or more articles subsequently formed in the mold.
As briefly alluded to above, the pressure applied to the melt stream varies over the cycle of the system. More specifically, the injection cycle includes an injection period during which substantial pressure is applied to the melt stream so as to force it into the article formation cavity, and into the space between the bore and the outer surface of the bushing. Thereafter, this injection pressure is reduced while (i) the article is solidified, (ii) the mold is opened, (iii) the article is removed from the mold, (iv) a skin is formed across the gate after the removal of the article and the "vestige" attached thereto, and (v) the mold is reclosed. During this lower pressure time period, some of the unsolidified melt material located between the bushing and the cold bore wall tends to flow back (i.e., "decompress") into the gate area. Accordingly, upon the next increase in pressure, this degraded material may be forced into the article formation cavity of the mold along with the new "shot" of virgin melt. This reciprocating pumping action (i.e., compressing melt material into the gap between the bore and the mold under pressure and then reducing the pressure so as to allow some of the material between the bore and the bushing to flow back into the gate area) takes place during each mold cycle. It also creates an undesirable potential for damaging, or ruining, many molded articles.
The foregoing is particularly problematic when it is desired to change the color of the melt. Specifically, during low pressure portions of the injection molding cycle, the above-described reciprocating pumping action tends to draw melt of the original color located between the outer surface of the heated bushing and the bore into the main melt stream of the new color. This ruins articles molded from the melts so combined. Thus, mold disassembly to remove the melt of the original color from the gap between the bushing and the bore in the mold is often required in order to effectuate a melt color change. Obviously, this is time consuming, inefficient and expensive.
Further, degradation of the solidified melt along the bore walls may cause pieces thereof to flake or break off so as to be available for migration into the gate area. Such flakes or particles tend to ruin a molded article and/or to clog the internal molding cavity if they are allowed to flow through the gate. Further, they may obstruct the gate itself. In the latter case, disassembly of the system is required to clean out the gate. Still further, the insulation (i.e., resistance to heat conduction) properties of the melt material are less effective than air, but more effective than metal. Accordingly, the use of such materials as insulators does not provide an optimum temperature gradient (profile) in the gate area of the system.
In view of the foregoing, various alternative solutions have been adopted in an attempt to solve the above-described problems. In one such alternative, the gap between the bushing and the bore has been filled with materials which exhibit high heat stability and resistance to thermal break down. These materials, however, still tend to crack and/or otherwise deteriorate under the forces of thermal expansion and contraction and/or under the applied pressures exerted thereupon by the melt flowing through the gate area. Hence, flakes or particles of such materials can enter the melt flow, and ruin or damage molded articles and/or obstruct the gate. Further, melt material may enter the cracks in such materials during the high pressure phase of the injection molding cycle so as to accelerate the break down thereof and reduce their insulative properties. Still further, no such material has yet been found which does not have significant thermal transmission (conductivity) properties. Thus, such materials tend to be less effective insulators than air, and to draw significant amounts of heat from the bushing toward the mold. Of course, this results in the need to supply additional heat input to the bushing, and reduces the efficiency of the apparatus.
Finally, solid mechanical seals (usually metallic in composition) have been used between the bushing and the bore in the region immediately surrounding the gate area. Such solid mechanical seals have also proven to be unsatisfactory. More particularly, such seals suffer from thermal and mechanical problems. For example, the thermal expansion coefficients and tolerances of such seals must be very carefully determined according to both the geometry of the apparatus into which they are to be placed and the operating parameters of that apparatus. If this is not done, or is done incorrectly, excessive forces may be applied to the gate area. Since the dimensions of the walls forming the gate area are typically quite small and injection pressures are fairly large, the application of such forces to such mechanical seals may "blow out" the gate area and/or the seal. Such a "blow out" requires substantial capital investment for replacement or repair, and/or significant down time to the molder. Further, such solid seals typically have significant heat conductivities and, therefore, cannot provide the desired temperature gradient (profile) for efficient gate operation.